Thermal Transport and Mechanical Stress Mapping of a Compression Bonded GaN/Diamond Interface for Vertical Power Devices

Abstract

Bonding diamond to the back side of gallium nitride (GaN) electronics has been shown to improve thermal management in lateral devices; however, engineering challenges remain with the bonding process and characterizing the bond quality for vertical device architectures. Here, integration of these two materials is achieved by room-temperature compression bonding centimeter-scale GaN and a diamond die via an intermetallic bonding layer of Ti/Au. Recent attempts at GaN/diamond bonding have utilized a modified surface activation bonding (SAB) method, which requires Ar fast atom bombardment immediately followed by bonding within the same tool under ultrahigh vacuum (UHV) conditions. The method presented here does not require a dedicated SAB tool yet still achieves bonding via a room-temperature metal−metal compression process. Imaging of the buried interface and the total bonding area is achieved via transmission electron microscopy (TEM) and confocal acoustic scanning microscopy (C-SAM), respectively. The thermal transport quality of the bond is extracted from spatially resolved frequency-domain thermoreflectance (FDTR) with the bonded areas boasting a thermal boundary conductance of >100 MW/m 2 •K. Additionally, Raman maps of GaN near the GaN−diamond interface reveal a low level of compressive stress, <80 MPa, in well-bonded regions. FDTR and Raman were coutilized to map these buried interfaces and revealed some poor thermally bonded areas bordered by high-stress regions, highlighting the importance of spatial sampling for a complete picture of bond quality. Overall, this work demonstrates a novel method for thermal management in vertical GaN devices that maintains low intrinsic stresses while boasting high thermal boundary conductances.

Measurements of Thermal Resistance Across Buried Interfaces with Frequency-Domain Thermoreflectance and Microscale Confinement

“…Many recent studies have focused on the development of techniques to characterize the thermal resistance across interfaces that are 10s to 100s of μm below the surface of a material. − Such interfaces are abundant in modern devices, including multilayer microelectronics packaging, , wide bandgap materials and devices, , power electronics architectures, ,, and memory storage systems, and are fast becoming the largest bottleneck to sufficient heat dissipation in the next generation of these applications. Critically, interfaces at these depths are extremely challenging to characterize using existing steady-state techniques, − which are limited to spatial resolutions above several hundred μm’s, or with advanced optical pump–probe thermoreflectance techniques, which probe at depths that range between nm’s to single-digit μm below a sample surface. , Most recent techniques have relied on augmentations to existing thermoreflectance systems; for instance, several studies achieve larger thermal penetration depths by extending the range of modulation frequencies applied to the pump beam. − In general, improvements in thermal penetration depth ( δ = 2 α / f ; α is thermal diffusivity, f is modulation frequency) have been limited to <10 μm due to spreading in the upper transducer layer …”